With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European SPS such as the proposed “Galileo” program. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.
The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver typically has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PRN) code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During the acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second (i.e. 1 bit/20 msec). This navigational data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames, frames and super-frames. A sub-frame consists of 300 bits of data and is transmitted for 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A super-frame consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is always the same and is known as TLM word and first eight bits of this TLM word are preamble bits used for frame synchronization. A Barker sequence is used as the preamble because of its excellent correlation properties. The other bits of this first word contains telemetry bits and is not used in the position computation. The second word of any frame is the HOW (Hand Over Word) word and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity with the last two bits of parity always being ‘0’s. These two ‘0’s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contains clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemeris are used to precisely determine the position of the GPS satellites. These ephemeris are uploaded every two hours and are valid for four hours to six hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and almanac of satellites 25 to 32. These almanacs are similar to the ephemeris but give a less accurate position of the satellites and are valid for six days. The 3 to 10 words of the sub-frame 5 contain only the almanacs of different satellites in different frames. The super frame contains twenty five consecutive frames. While the contents of the sub-frames 1, 2 and 3 repeat in every frame of a superframe except the TOW and occasional change of ephemeris every two hours. Thus the ephemeris of a particular signal from a satellite contains only the ephemeris of that satellite repeating in every sub-frame. However, almanacs of different satellites are broadcast in-turn in different frames of the navigation data signal of a given satellite. Thus the 25 frames transmit the almanac of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanac is included in the sub-frame 4. The almanacs and ephemeris are used in the computation of the position of the satellites at a given time.
Accordingly, it is clear from the foregoing that the process of locking onto and synchronizing to signals from positioning system satellites, and particularly to being able to extracting meaningful data from such signals, is an important process before determining position and navigating using such signals can begin.
One problem that often makes synchronization difficult and time consuming is when signals from satellites are weak. More particularly, the received signal is characterized by the carrier to noise density ratio C/N0 having units of dB-Hz (sometimes also referred to as CNO). A weaker signal has a lower value of C/N0. At low values of C/N0 there are difficulties in the synchronizations of carrier and frame. Synchronization of code and bit can be maintained down to C/N0 values of about 20 dB-Hz. (All the C/N0 values stated here correspond to the received signal at the output to the correlator.) Synchronization of the carrier phases can be maintained only up to C/N0 values of about 30 dB-Hz by a phase lock loop (PLL). There is a way out for lower values of C/N0. Instead of maintaining synchronization of carrier phases synchronization of carrier frequencies can be maintained down to C/N0 values of lower than 15 dB-Hz by an automatic frequency control (AFC) loop. Data demodulation is then done by differential techniques using both the in-phase (I) and quadrature-phase (Q) samples. However, successful data demodulation can only be done down to C/N0 values of about 26 dB-Hz.
For lower values of C/N0 (i.e. weak signals), data demodulation is unreliable and so some techniques have been developed to perform frame synchronization by correlating received signals with known bits in the TLM word of each subframe (e.g. the 8-bit preamble). These techniques can permit frame synchronization to be achieved down to about 21 dB-Hz. However, they are time consuming because the TLM word only occurs every 6 seconds at the beginning of each sub-frame, and successful correlation often requires multiple iterations of these words.
Because conventional synchronization schemes were sometimes time-consuming, an approach called “Sync Free Nay” has sometimes been used. However, this approach typically requires acquiring and tracking signals from five satellites, which is not always possible. Moreover, using sync free nav sometimes leads to a decrease in initial position performance because sync free nav can have a time offset that leads to position error due to satellite motion.
Accordingly, a method and apparatus for quickly and effectively synchronizing to signals from positioning system satellites remains desirable, whether or not sync free nav is also used. Put another way, a need remains for frame sync methods that can operate successfully with very weak GPS signals (≦15 dB-Hz), and also fast enough (time to frame sync) that standards performance (e.g. 3GPP) and user experience are maintained or improved in the no sync-free nav case.